fix(sheetset): wasm build, lock files and sheet set field refresh

- `.dst` read/write go through native-only helpers (a browser answers
  "Not available"); the folder pickers are web no-ops; time stamps use
  iced::time (web-safe).
- Lock files are written only for drawings that are sheets of an open set,
  released when the last set closes, and a lock a crashed session left
  (recorded with its process id, same user and machine) is taken back.
  The `.dwl2` header is well-formed and the date line comes from the OS
  long date on every platform. The 1 Hz poll runs only while a set is open.
- Refreshing sheet set fields never stores `####`: fields no open set
  resolves keep their value, and only the hosts that changed are redrawn
  (and their tab marked modified).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/view/mod.rs

@@ -2643,12 +2643,15 @@ impl OpenCADStudio {
         };
         #[cfg(target_arch = "wasm32")]
         let plugin_drain = Subscription::none();
-        // Open sheet sets reload when their .dst changes on disk.
+        // While a sheet set is open: it reloads when its .dst changes on disk,
+        // and its sheets' .dwl lock files follow the open tabs.
         #[cfg(not(target_arch = "wasm32"))]
-        // Open sheet sets reload when their .dst changes on disk; the
-        // drawings' .dwl lock files follow the open tabs.
-        let sheet_set_poll = iced::time::every(std::time::Duration::from_secs(1))
-            .map(|_| Message::SheetSet(crate::ui::window::sheet_set::SheetSetMsg::Poll));
+        let sheet_set_poll = if self.sheet_set.sets.is_empty() {
+            Subscription::none()
+        } else {
+            iced::time::every(std::time::Duration::from_secs(1))
+                .map(|_| Message::SheetSet(crate::ui::window::sheet_set::SheetSetMsg::Poll))
+        };
         #[cfg(target_arch = "wasm32")]
         let sheet_set_poll = Subscription::none();
         let hatch_pattern_keys = if self.tabs[self.active_tab]